Large-scale environmental signals in seagrass blue carbon stocks are hidden by high variability at local scales

Increasing focus on nature-based climate change mitigation and adaptation strategies has led to the recognition of seagrasses as globally significant organic carbon (C_org) stocks. However, estimates of carbon stocks have been generally confined to a few regions, with few African studies represented in global datasets. In addition, the extent to which biogeographical and environmental variation shape carbon stocks in marine vegetated environments remains uncertain. For South Africa, Zostera capensis is the dominant seagrass species with limited mapping and quantification of its C_org stocks. Here, we measured Z. capensis C_org stocks at six South African estuaries spanning ∼1800 km of the cool-temperate to subtropical marine environmental gradient. Targeting the intertidal zone of the upper and lower estuary reaches, we collected Z. capensis sediments to a depth of 50 cm and measured the C_org, with the median C_org stock estimated at 24.11 Mg C ha^-1 (40.4 ± 53.02; mean ± SD). While this is lower than the global average, these data demonstrate that Z. capensis ecosystems are important contributors to blue carbon stocks in the region. Measured C_org stocks showed significant differences between sampling sites for estuaries; however, we did not detect significant differences between estuaries due to high intra-estuarine C_org variability. Examination of biogeographical regions, terrestrial and marine environmental variables as drivers of C_org variability revealed that annual mean sea surface temperature may explain variation in C_org stocks. Furthermore, we found evidence of signals of biogeographical regions and precipitation driving some of the variability in C_org stocks; however, this requires further investigation. Overall, our estimates for Z. capensis add to ongoing national and global efforts to quantify seagrass C_org stocks across environmental and biogeographic gradients to better determine their contributions as nature-based solutions to climate change.